<?php

abstract class Process {

    protected $source;
    abstract function read();
    abstract function parse($type = "txt");

    protected function __construct($source) {
        $this->source = $source;
    }

}

class File extends Process {

    protected function __construct($source) {
        parent ::__construct($source);
    }

    public function read() {
       $fileHandle = fopen($this->source, "r");
       while (!feof($fileHandle)) {
           $line = fgets($fileHandle);
           echo $line;
       }
       fclose($fileHandle);
    }

    public function parse($type="txt") {
    switch($type) {
        case "txt" :
            // parse txt file
        break;
        case "xml" :
            // parse xml file
        break;
    }
}

$file = new File("file.txt", "some file resource");
$file->read();